1. Field of the Invention
The present invention relates generally to a mold assembly of an injection-molding machine adapted to mold discs such as optical discs or similar products, and more particularly to improvements in the mold assembly for prtecting a stamper against thermal stressed and gases trapped therein, so as to assure stable molding of the products with excellent quality.
2. Discussion of the Prior Art
A known mold assembly for injection molding of discs such as optical and magnetic discs like audio discs, video discs and memory discs uses a stationary die, and a movable die which incorporates a stamper retainer thereon and which is adapted to move toward and away from the stationary die. The stationary and movable dies cooperate to define a mold cavity corresponding to a profile of the product to be molded cavity, so that information signals recorder on one surface of the stamper are reproduced on a corresponding surface of an injection-molded disc.
In the disc-forming mold assembly of the type indicated above, the stamper which usually takes the form of a doughnut is positioned within the mold cavity such that the outer and inner peripheral portions of the stamper are retained on the movable die by respective retainer rings.
Described more particularly referring to FIGS. 7 and 8, the outer peripheral portion 71 of the stamper 70 is bound by the outer retainer ring 65 provided on the movable die block 60, while the inner peripheral portion of the stamper 70 is similarly bound by the inner retainer ring (not shown). Thus, the stamper 70 is held in position with respect to the movable die block 60.
The movable die block 60 has a mirror surface 61 on which the stamper 70 rests at one of its opposite surfaces. The other surface of the stamper 70 serves as a stamping surface which is accurately formed with information signals recorded thereon.
With the thus arranged movable die block 60 moved to the stationary die block 50, the mold cavity C is defined by the stamper 70, the outer retainer ring 65, and a mirror surface 51 of the stationary die block 50. The resin material is injected into the mold cavity C, whereby a disc is formed such that the information signals on the stamping surface of the stamper 70 are reproduced on one of opposite surfaces of the disc.
The stamper 70 having the information signals recorded thereon is reproduced from a master pattern, by means of a metal electroforming, for example. In an injection molding operation, the metal stamper 70 is heated due to heating of the mold assembly to an adjusted operating temperature, and due to heat of a melt of the injected resin material which fills the mold cavity. Consequently, the stamper 70 undergoes thermal expansion. Further, flows of the molten resin within the cavity C cause the stamper 70 to be subject to a tensile force in the direction parallel to its plane, i.e., in the radially outward direction. Thus, the stamper 70 tends to be elastically elongated or expand in the radially outward direction. To facilitate such thermal expansion and elastic elongation of the stamper 70, there is provided a clearance (indicated at 68 in FIG. 8), usually on the order of several microns, between the stamper 70 and a flange portion 66 of the outer retainer ring 65.
Generally, the periphery of the stamper 70 is shaped by a blanking operation with a punch and die. In this blanking operation, the stamper 70 more or less has a warpage or buckling 73, or burr on its one side, as indicated in FIG. 8, due to the blanking force exerted at its edge. Reference character 81 in FIG. 8 indicates a space provided between the outer periphery of the stamper 70 and the outer retainer ring 65, to accomodate radial expansion of the stamper 70. The warpage 73 at the outer peripheral portion 71 of the stamper 70 prevents a radially outward movement of the stamper 70 when the stamper 70 undergoes thermal expansion and elastic deformation as indicated above. In this condition, the stamper 70 tends to be locally flexed or curved as shown in FIG. 7, and create an air space A between the mirror surface 61 of the movable die block 60 and the corresponding surface of the stamper 70. The air space A undesirably functions as a thermal insulator, and may cause inefficient cooling of the formed resin material at its portion which corresponds to the flexed portion of the stamper 70. Thus, the flexture of the stamper 70 leads to dimensional inaccuracy of the molded optical disc, which may induce optical scanning or reading errors of the disc.
Another problem encountered in the known mold assembly as shown in FIGS. 7 and 8 is derived from gases such as monomer gases which are produced during injection of the resin material into the mold cavity C. More specifically, the gases flow through a small gap between the stamper 70 and the flange portion 66 of the retainer ring 65, and enter into an annular void S1 in which the outer peripheral portion of the stamper 70 is positioned, or an annular void (not shown) in which the inner peripheral portion of the stamper 70 is positioned. When the molded product is removed from the mold assembly, the stamper 70 is pulled a small distance away from the mirror surface 61 of the movable die block 60. At this time, the gases trapped in the above-indicated voids enter between the stamper 70 and the mirror surface 61. As a result, the mirror surface 61 and the corresponding rear surface of the stamper 70 are subject to electrolytic corrosion by the gases.
Since the stamper 70 is a thin plate having a thickness of about 0.25-0.3 mm, corroded areas of the stamper are simulated as corrosion marks on the molded product, and the quality of the product is deteriorated. Where the product is an optical disc, in particular, slight surface strains of the disc have a direct adverse effect on the quality of the disc. For this reason, the stamper 70 and the movable die block 60 of the conventional mold assembly must be cleaned, replaced or re-machined at comparatively short intervals (e.g., every 20,000-30,000 shots).